A device with gain has the property of:

A device that exhibits 'gain' increases the power, voltage, or current of a signal passing through it. This fundamental process of boosting a signal's strength is known as amplification. Gain is simply the quantitative measure of how much amplification is taking place. Conversely, attenuation is the exact opposite of amplification (reducing the signal). Oscillation is the generation of a continuous waveform, and modulation is the encoding of information onto a carrier. None of these other terms define the property of having gain.